It is known that the efficiency or efficacy of a catalytic converter treating the exhaust gas of an engine is significantly effected by the ratio of air to fuel supplied to the engine. At stoichiometric ratio, catalytic conversion efficiency is high for both oxidation and reduction conversions. The air/fuel stoichiometric ratio is defined as the ratio of air to fuel which in perfect combustion would yield complete consumption of the fuel. The air/fuel ratio LAMBDA of an air/fuel mixture is the ratio of (a) the amount by weight of air divided by the amount by weight of fuel actually being fed to the engine at a point in time, to (b) the air/fuel stoichiometric ratio. Closed loop fuel control systems are known for use in keeping the air/fuel ratio in a narrow range about the stoichiometric ratio, known as a conversion window. Closed loop fuel control systems are known utilizing one EGO sensor located before and one after the catalytic converter to maintain the air/fuel ratio inside the conversion window.
It is also known that the efficiency or efficacy of catalytic conversion is affected by the oxygen storage capability of the catalytic converter. A properly operating catalytic converter dampens oxygen concentration fluctuations in the exhaust stream. A system for testing catalytic converter efficiency proposed in SAE paper No. 900062, Detection of Catalyst Performance Using On-Board Diagnostics, employs two exhaust gas oxygen ("EGO") sensors, one upstream and one downstream of the catalytic converter, to detect oxygen content in the exhaust gas. The system employs test signals in the form of an air/fuel ratio swing on both sides of stoichiometry at predetermined rates or frequencies caused by fuel control system perturbations. By comparing the change in response patterns between the upstream and downstream EGO sensors, a determination can be made about catalytic converter efficacy.
A method is needed for accurately and reliably determining catalytic converter efficiency, especially in an on-board system, preferably a method which is suitable for incorporation into an overall fuel control system for the engine.